In vitro studies demonstrate that N-demethylation of amitriptyline to nortripty-line is mediated mainly by cytochrome P450-3A4, while hydroxylation of amitripty line to 10-hydroxy-amitriptyline is mediated in part by P450-2D6. This data predicts the possibility of impaired clearance of amitriptyline in vivo by coadministration of ketoconazole or quinidine, highly active and relatively specific competitive inhibitors of 3A4 and 2D6, respectively. This hypothesis will be tested in two prospective double-blind controlled kinetic-dynamic studies.